Problem 116 - Chemistry Matter and Change

Question

What is the difference between a covalent molecular solid and a covalent network solid? Do their physical properties differ? Explain your answer.

Step-by-Step Solution

Answer

Covalent molecular solids are soft with low melting points due to weak intermolecular forces, while covalent network solids are hard with high melting points due to strong covalent bonds.

Step 1: Understanding Covalent Molecular Solids

Covalent molecular solids are composed of molecules held together by weak intermolecular forces such as van der Waals forces or hydrogen bonds. These solids typically have low melting points, are soft, and volatile due to the weak forces between molecules. Examples include ice and dry ice.

Step 2: Understanding Covalent Network Solids

Covalent network solids consist of atoms connected by strong covalent bonds in a continuous network throughout the solid. These solids are characterized by high melting points, hardness, and are typically non-conductive. Examples of covalent network solids are diamond and quartz.

Step 3: Comparing Physical Properties

Covalent molecular solids are characterized by soft textures and low melting points due to the weak intermolecular forces. In contrast, covalent network solids are hard, strong, and have high melting points because of the robust network of covalent bonds.

Step 4: Summary of Differences

The primary difference between the two is the nature of bonding: molecular solids are held by weak intermolecular forces, whereas network solids are held by strong covalent bonds. Their physical properties differ due to these bonding differences, where molecular solids are softer and have lower melting points compared to the hard and high-melting network solids.

Key Concepts

Molecular Solids

Molecular solids are a fascinating category of solids where atoms within each molecule are bonded covalently, but molecules themselves are held together by relatively weak forces, known as intermolecular forces. These forces do not involve sharing or transferring electrons between atoms but are based on attractions between molecules.
This structural arrangement results in several distinctive properties:

• Low melting and boiling points - the weak forces between molecules require less energy to overcome.
• Soft or brittle nature, often leading to easy breakability.
• Poor conductivity of electricity - since there are no free-moving charged particles.
• Higher volatility - because molecules can easily separate from one another.

It’s useful to differentiate between these solids and other types by recognizing that their key interactions are intermolecular, distinguishing them from solids like network covalent structures.

Network Solids

Network solids, also known as covalent network solids, present a sharp contrast to molecular solids. These are composed of a continuous three-dimensional network of covalently bonded atoms. Each atom is bonded to multiple neighboring atoms through strong covalent bonds, creating a robust structure.
This bonding accounts for some of their unique characteristics:

• Very high melting points due to the immense energy required to break the strong covalent bonds.
• Extreme hardness and rigidity, making them difficult to fracture.
• Generally poor electrical conductivity under normal conditions - although this varies (graphite is an exception).
• Low volatility - as the atoms are strongly bonded in place.

Understanding these properties allows us to anticipate the behaviors of these solids in diverse applications. For instance, diamonds, a type of network solid, are ideal for cutting tools due to their hardness.

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles (e.g., atoms or ions). These forces, although weak compared to covalent or ionic bonds, play a critical role in determining the physical properties of molecular solids.
Major types of intermolecular forces include:

• Van der Waals forces - these include dipole-dipole interactions, London dispersion forces (present in all molecular solids), and hydrogen bonds (the strongest among them).
• Hydrogen bonds - strong types of dipole-dipole attractions involving hydrogen atoms bonded to highly electronegative atoms.

While these forces do not contribute to the hardness and scalability of network solids, they crucially influence the melting, boiling points, and solubility of molecular solids. Understanding these forces aids in predicting behaviors of different molecular substances in chemical and physical contexts.